The design of orthopedic implants has progressed through the development of plates or other stabilization means, including for example rods and or mesh, intended for application to specific bones, and for use with characteristic types of fractures. These plates are optimally designed to correspond to a generalized shape of the bone and further include fixation means, which are generally screws or pegs, that hold the plate to the bone, and further hold fragments of the bone to the plate in association with other fragments so that the fragments will fuse together.
In position on the bone, the plate, fasteners and bone form a construct that accepts dynamic loading. The interaction of the screws or pegs with the bone, and with the plate is a complex matter. Typically, screws include a threaded shaft and pegs may include a threaded shaft or may simply be cylindrical and be devoid of threads about the shaft. Bone has a hard cortical surface and a porous cancellous internal portion and the variable nature of bone must be taken into consideration in the design of an implant system. Further, as bone lives, it reacts to loading and to motion of the fasteners so that threads can loose purchase over time as the bone shifts away from the threads. Further loading between fragments influences fusion of the fragments. Thus, the design of the plate/fastener interface includes considerations of loading as well as accommodation of typical patterns of fragmentation and the intention to capture cortical surfaces and to avoid surrounding soft tissue.
Typically, locking screws or pegs, are locked into a relationship with a plates, and may also lock the bone into that relationship, or in the event that the peg does not include a threaded shaft, the peg may act more as a support for a fragment. Alternatively, if the screw or peg is threaded about its shaft, it may lock the bone fragment, and in the event that the relationship with the plate is not a locking relationship, the fastener may have a certain amount of play relative to the plate, which further accommodates variations in angles that a surgeon may wish to achieve, but which does not inhibit the fastener from backing out of the plate. Fasteners, which lock into the plate, are less likely to back out and provide proud surfaces that could cause irritation to surrounding soft tissue.
There are advantages to providing locking fasteners at selected angles that are designed to capture typical fragments and secure their relationship to the plate. However, it is even more advantageous to provide these locking fasteners with locking fasteners that can be inserted at variable angle and subsequently locked into that position. While other systems have provided locking mechanisms that allow a variable angle fastener to be inserted into a threaded locking hole, these systems do not provide acceptable holding power of the angle of the variable angle fasteners. The present invention meets that need by providing a plate and locking and variable locking fasteners, the plate having a first set of at least one fastener opening that is set in the plate at a selected angle through the plate and that is internally threaded to accept the externally threaded head of a locking fastener, and the plate further having a second set of at least one fastener opening that is not internally threaded, but which has a concavely rounded internal surface that mates with a corresponding convexly rounded external surface of a locking insert at a selected variable angle, the locking insert accepting a fastener and wherein the fastener has means to lock the insert at the selected variable angle relative to the plate. As examples of the locking means, the locking insert may include one or more expansion slots that allow the locking insert to be expanded in the opening in the plate so as to secure it in the plate. More particularly, the locking insert may also include a cam raceway that accepts a camming member on the head of the fastener that causes the locking insert to expand radially, or the locking insert may include internal threads that mate with external threads of the fastener. In this instance, the fastener used in the fixed locking openings may be the same fastener that is used in the variable locking opening, which provides an inventory advantage (it requires fewer fastener types for the surgery).
The present invention can be used in any number of surgical applications, including for example, for small bone plates such as radial plates, tibial plates, and calcaneal plates; for long bone plates, and for the spine or pelvis.